Control of texture and morphology of zinc films through pulsed methods from additive-free electrolytes

ABSTRACT

Various aspects according to the instant disclosure relate to a method of electrodeposition of zinc. The method includes independently controlling at least one of an electrical peak current and a duty cycle. The method further includes depositing the zinc on a substrate.

CLAIM OF PRIORITY

This patent application claims the benefit of U.S. Provisional Patent Application No. 63/200,787, tiled Mar. 29, 2021, which is incorporated by reference herein in its entirety.

BACKGROUND/ND

Zinc can be applied to a substrate for a variety of different applications. The manner in which zinc is applied can affect the properties of the zinc for a particular application.

SUMMARY OF THE INVENTION

Various aspects according to the instant disclosure relate to a method of electrodeposition of zinc. The method includes independently controlling at least one of an electrical peak current and a duty cycle. The method further includes depositing the zinc on a substrate.

Various aspects according to the instant disclosure relate to a method of electrodeposition of zinc. The method includes independently controlling at least one of an electrical peak current and a duty cycle. The electrical peak current has a density in a range of from about 0.01 A/cm² to 156 A/cm² or about 0.02 A/cm² to 1.5 A/cm² and the duty cycle is in a range of from about 0.1% to about 90% or about 2% to about 10%. The method further includes depositing the zinc on a substrate, wherein the zinc layer comprises a plurality of needle-shaped structures, hexagonal-plate structures, or a mixture thereof.

Various aspects according to the instant disclosure relate to an assembly. The assembly includes a steel substrate. The assembly further includes a zinc layer deposited about the steel substrate, wherein the zinc layer comprises a plurality of needle-shaped structures, hexagonal-plate structures, or a mixture thereof.

BRIEF DESCRIPTION OF THE FIGURES

The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments of the present invention.

FIG. 1 is shows generic pulse waveform used for the electrodeposition of Zinc with a forward cathodic bias, followed by an off-time to allow replenishment of the diffusion layer.

FIG. 2 shows SEM images of the electroplated zinc samples on stainless steel substrates as a function of applied current density with other variables constant at the same magnification (scale bar included). Inset shows the optical image of the same deposits.

FIG. 3A shows X-ray diffraction patterns for the electrodeposited samples on stainless steel substrates as a function of applied peak current density with constant charge density and duty cycle including database standards for comparison.

FIG. 3B shows crystallite size of electrodeposited zinc samples for the same samples.

FIG. 4A shows variation in absolute texture coefficient for zinc electrodeposited on stainless steel substrates.

FIG. 4B shows variation in relative texture coefficient (RTC) of electrodeposited zinc samples on stainless steel substrates as a function of applied current density with constant charge density and duty cycle.

FIG. 5 shows a schematic of a hexagonal zinc plate indicating the directional planes of growth with the lowest energy basal plane (002) and the high energy (100) plane perpendicular to each other.

FIG. 6 shows a series of SEM images of the electroplated zinc samples on stainless steel substrates as a function of duty cycle with constant time on, and charge density at the same magnification, Inset shows the higher magnification images of the same samples.

FIG. 7A shows variation in crystallite size of electroplated zinc samples on stainless steel substrates.

FIG. 7B shows variation in relative texture coefficient (RTC) of electrodeposited zinc samples on stainless steel substrates as a function of duty cycle, while holding the peak current and charge density constant.

FIGS. 8A-8C show SEM images of zinc-coated carbon steel substrates generated at 0.1% duty cycle.

FIGS. 8D-8F show SEM images of zinc-coated carbon steel substrates generated at) 50% duty cycle.

FIG. 9A shows XRD patterns of the zinc-coated carbon steel substrates generated using low (0.11%) and high duty cycle (50%) along with relevant database standards.

FIG. 9B shows distribution of texture coefficients demonstrating the dominance of (002) textures at low duty cycle for zinc-coated carbon steel substrates.

FIG. 10A shows Polarization curves for zinc coated carbon steel substrates produced at low and high duty cycle in a de-aerated 0.1 M aqueous NaCl electrolyte.

FIG. 10B shows corrosion rates for zinc coated carbon steel substrates produced at low and high duty cycle in a de-aerated 0.1 M aqueous NaCl electrolyte.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to certain embodiments of the disclosed subject matter, examples of which are illustrated in part in the accompanying drawings. While the disclosed subject matter will be described in conjunction with the enumerated claims, it will be understood that the exemplified subject matter is not intended to limit the claims to the disclosed subject matter.

Throughout this document, values expressed in a range format should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a range of “about 0.1% to about 5%” or “about 0.1% to 5%” should be interpreted to include not just about 0.1% to about 5%, but also the individual values (e.g., 1%, 2%, 3%, and 4%) and the sub-ranges (e.g., 0.1% to 0.5%, 1.1% to 2.2%, 3.3% to 4.4%) within the indicated range. The statement “about X to Y” has the same meaning as “about X to about Y,” unless indicated otherwise. Likewise, the statement “% about X, Y, or about Z” has the same meaning as “about X, about Y, or about Z,” unless indicated otherwise.

In this document, the terms “a,” “an,” or “the” are used to include one or more than one unless the context clearly dictates otherwise. The term “or” is used to refer to a nonexclusive “or” unless otherwise indicated. The statement “at least one of A and B” or “at least one of A or B” has the same meaning as “A, B, or A and B.” In addition, it is to be understood that the phraseology or terminology employed herein, and not otherwise defined, is for the purpose of description only and not of limitation. Any use of section headings is intended to aid reading of the document and is not to be interpreted as limiting; information that is relevant to a section heading may occur within or outside of that particular section. All publications, patents, and patent documents referred to in this document are incorporated by reference herein in their entirety, as though individually incorporated by reference. In the event of inconsistent usages between this document and those documents so incorporated by reference, the usage in the incorporated reference should be considered supplementary to that of this document; for irreconcilable inconsistencies, the usage in this document controls.

In the methods described herein, the acts can be carried out in any order without departing from the principles of the invention, except when a temporal or operational sequence is explicitly recited. Furthermore, specified acts can be carried out concurrently unless explicit claim language recites that they be carried out separately. For example, a claimed act of doing X and a claimed act of doing Y can be conducted simultaneously within a single operation, and the resulting process will fall within the literal scope of the claimed process.

The term “about” as used herein can allow for a degree of variability in a value or range, for example, within 1.0%, within 5%, or within 1% of a stated value or of a stated limit of a range, and includes the exact stated value or range. The term “substantially” as used herein refers to a majority of, or mostly, as in at least about 50%, 60%, 70%, 80%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5%, 99.9%, 99.99%, or at least about 99.999% or more, or 100%. The term “substantially free of” as used herein can mean having none or having a trivial amount of, such that the amount of material present does not affect the material properties of the composition including the material, such that about 0 wt % to about 5 wt % of the composition is the material, or about 0 wt % to about 1 wt %, or about 5 wt % or less, or less than or equal to about 4.5 wt %, 4, 3.5, 3, 2.5, 2, 1.5, 1, 0.9,0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, 0.01, or about 0.001 wt % or less, or about 0 wt %.

The electrodeposition of Zinc has been investigated with extensive commercial application for the protection of steel against corrosion. Traditionally, the direct current (DC) method of electroplating has been explored in terms of varying current density, alone, to obtain variations in morphology, grain size, preferred crystalline orientation, which in turn have significant consequences for its properties such as corrosion resistance, hardness, ductility, contact angle, contact resistance, and porosity. More recent efforts have also included the use of pulsed waveforms to achieve several of the same effects in grain size, morphology, and crystalline orientation.

The use of pulse methods of deposition have been investigated for a variety of materials including zinc oxide, its alloys, in both aqueous and non-aqueous electrolytes. However, both DC and pulsed methods, so far, have relied on a variety of secondary additives, in addition to the Zn²⁺ source and a supporting electrolyte.

Specific examples of such additives include polymers, surfactants, secondary metal ions, arene-based brighteners and quaternary ammonium ion salts. Such secondary additives have been demonstrated to be necessary to achieve the changes to several properties of the deposit, chiefly, the crystalline orientation through selective adsorption on growing facets. The use of such additives has its disadvantages as follows: They can be (i) complex, expensive, and even be consumed at the cathode requiring replenishment at regular intervals of the plating process. Furthermore, (ii) such additives have also been reported to be incorporated into the deposit, which can lead to a significant variation in the physical properties of the deposit and hence highly undesirable. Also, from an environmental perspective, (iii) the electroplating of Zinc from which is done typically from both alkali and acidic baths, have deleterious effects due to effluent discharge. While these are both more environmentally friendly than the historically used cyanide baths, they still involve corrosive elements with long term environmental concerns and sustainability challenges.

Grain size and morphology of zinc have a significant effect on mechanical and catalytic properties of the zinc. For example, mechanical properties such as hardness, yield strength, tensile strength, and impact resistance improve with decreasing grain size. The interlocking of such smaller grains throughout the material is attributed to enhancement properties as compared to larger grains. Similarly, the density of grain boundaries has been demonstrated to have an effect on catalytic properties during the conversion of carbon dioxide (CO₂) to useful chemicals and fuels. Furthermore, the crystalline orientation or texture of Zinc deposits has also been of considerable interest. Recent studies have demonstrated that from a catalysis perspective, the (101) texture has superior catalytic performance for the conversion of CO₂, as compared to the (002) texture, as determined by DFT studies. Additional studies have also studied the variation in corrosion resistance of Zinc achieved by varying the texture and morphology of the deposit on steel substrates. Hence, there is a need to explore simple methods of achieving the desired grain size, and texture in such deposits of zinc without any convoluting effects of additives.

The pulse plating process involves a number of variables to enhance control over the surface chemistry and mass transfer dynamics of electrochemical processes. FIG. 1 shows an example of a typical waveform, consisting of a cathodic (forward) pulse followed by a relaxation period (off-time). The cathodic peak current (i_(c)), cathodic on time (t_(c)), and the relaxation-time (t_(o)) are individual variables for process control. Furthermore, anodic pulses can also been added to such an electroplating process, introducing two more variables, e.g., anodic peak current (i_(c)), cathodic on time (t_(c)). In such pulse and pulse reverse processes, there are unlimited combinations of peak current densities, duty cycles, and frequencies to obtain a given electrodeposition rate, allowing access to a wider parameter space and consequently the properties of the deposit. In conventional direct current (DC) electrochemical processing, the current is turned on and held for the duration of the process. By interrupting this constant current, as in the pulsed process, one may achieve results not possible with conventional DC techniques. In the case of electroplating, these include control of alloy composition, nucleation densities, and microstructure, as well as the use of simplified plating chemistries.

The electrodeposition techniques described herein include disposing a substrate in an additive-free aqueous electrolyte bath that includes zinc salts. The electrolyte bath is maintained at a pH in a range of from about 4 to about 7, about 5 to about 6, less than, equal to, or greater than about 4, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8, 5.9, 6, 6.1,6.2,6.3,6.4,6.5,6.6,6.7, 6.8, 6.9, or about 7. As used herein, “additive free” means that the electrolyte bath is free of additives (e.g., surfactants, tetrabutyl ammonium, pyridinium salts, and the like) and/or a corrosive chemical (e.g., an alkaline or an acid).

Following disposing of the substrate into the bath, an electrical current is applied to the bath. The electrical peak current and the duty cycle are independently controlled to control the resulting morphology of the zinc on the substrate. For example, with respect to the electrical peak current, a density of the electrical peak current can range from about 32 mA/cm² to 156 A/cm², about 50 m A/cm² to 2 A/cm², about 0.02 A/cm² to 1.5 A/cm², less than, equal to, or greater than about 32 mA/cm², 50 mA/cm², 0.5 A/cm², 1.5 A/cm², 2 A/cm², 10 A/cm², 50 A/cm², 100 A/cm², or 156 A/cm². In addition to, or instead of, varying the peak current, a duty cycle can be adjusted. A “duty cycle” as used herein refers to the time a current is applied to the total duration of the cycle (e.g., t_(ON)/(t_(ON)+t_(OFF))). The duty cycle is expressed as a percent value. The duty cycle can range from about 0.1% to about 90%, about 2% to about 10%, less than, equal to, or greater than about 0.1%, 5, 10, 15, 20, 25, 30, 35, 40, 45, or 50%.

Controlling the peak current, duty cycle, or both determines the morphology of the zinc deposited on the substrate. For example, certain combinations of peak current density and duty cycle can yield needle-shaped structures of zinc deposited on the substrate while other combinations can yield hexagonal-plate structures of zinc deposited on the substrate. For example, a grain size of the zinc can be in a range of from about 5 nm to about 50 nm, about 15 nm to about 30 nm, less than, equal to, or greater than about 5 nm, 10, 15, 20, 25, 30, 35, 40, 45, or about 50 nm. Grain size and morphology of zinc have a significant effect on mechanical and catalytic properties of the zinc. For example, mechanical properties such as hardness, yield strength, tensile strength, and impact resistance improve with decreasing grain size.

Moreover, control over the peak current, duty cycle, or both can affect the orientation of the zinc in the zinc layer. For example, the zinc particles can be oriented about their respective longitudinal axis in substantially the same direction, the zinc particles can lie in substantially the same plane, or a combination thereof.

An average thickness of the zinc layer can be in a range of from about 5 micrometers to about 50 micrometers, about 10 micrometers to about 20 micrometers, less than, equal to, or greater than about 5 micrometers, 10, 15, 20, 25, 30, 35, 40, 45, or about 50 micrometers. The zinc layer can be applied to any degree of the total surface area of the substrate. For example, the zinc layer can be applied to about 5 percent of the total surface area of the substrate to about 100 percent of the total surface area of the substrate, about 20 percent of the total surface area of the substrate to about 90 percent of the total surface area of the substrate, less than, equal to, or greater than about 5 percent of the total surface area of the substrate, 10, 20, 30, 40, 50, 60, 70, 80, 90, or about 100 percent of the total surface area of the substrate.

The substrate includes steel. Examples of suitable steels include stainless steel, carbon steel, or a mixture thereof. An example of a carbon steel includes a series of alloys of carbon and iron containing up to about 1% carbon and up to 1.65% Mn. The substrate can conform to many suitable shapes. For example, the substrate can be substantially planar, curved, undulating, or any other desired shape.

EXAMPLES

Various embodiments of the present invention can be better understood by reference to the following Examples which are offered by way of illustration. The present invention is not limited to the Examples given herein.

Example 1

Example 1 describes the electrodeposition of zinc on stainless steel substrates using pulsed methods from additive-free electrolytes in an industrially scalable parallel plate flow cell. It is demonstrated that variables such as peak current and duty cycle can be used to independently control the morphology and preferential texture of the crystalline structure, which is in turn desirable for a variety of applications. Specifically, we have observed a transition of the preferential (002) facets at low peak current density (or low duty cycle) to (101) preferring facets at higher peak current density (or high duty cycle) as determined by analysis of texture coefficients. Further analysis using scanning electron microscopy reveals a transition from the conventional hexagonal-shaped particles of zinc to needle-shaped structures, accompanied by a change in average crystallite size from 33 nm to 19 nm, determined from X-ray diffraction studies. The variation in morphology is also correlated to crystallographic textures through analysis of the texture coefficients and an understanding of crystallization mechanisms occurring during deposition. Collectively, these results demonstrate that the morphology and crystalline orientation of zinc can be easily manipulated through tuning of the pulse parameters without the use of complex and expensive additives that have been traditionally used to achieve similar effects.

The inter-dependence of the variables involved in the electrodeposition process and decoupling of those variables is a considerable challenge. Such variables include the concentration of active ions, supporting electrolytes, additives, temperature, hydrodynamics, cell geometry, and additional pulse plating parameters explored herein. This example explores the effects of two pulse variables: (i) applied peak current density and (ii) the duty cycle on the resulting deposits while keeping all other parameters constant. As shown in the generic waveform in FIG. 1, the peak current density represents the initial period, where a cathodic bias is applied on the working electrode resulting in the deposition of ions from the solution (see equation 1). Since deposition was done using aqueous solutions, the hydrogen evolution reaction (HER) is also present as per equation 2.

Zn ²⁺ +Ze ⁻ −>Zn(s)  (1)

2H ⁺+2e ⁻ −>H ₂(g)  (2)

In acidic chloride baths, which have been used extensively, the HER heavily influences the deposition process given the large concentration of protons. Under the given conditions of near-neutral pH, such effects are expected to be comparatively less. Nevertheless, based on the values of the thermodynamic reduction potential, there are no pH conditions where the zinc deposition will be thermodynamically more favorable as compared to the HER. Since no additives were used in this study, the decoupling and interdependence of the deposition parameters are more straight forward. The observed changes to morphology and texture can better understood in correlation with the plating parameters being varied, which has been a challenge in similar studies that utilize additives. Cyclic voltammetry was used to understand the V-I behavior occurring during the deposition. It shows a typical cathodic peak for the deposition of Zn (II) ions form solution as well as an anodic peak for the stripping of Zinc metal into the solution. Both peaks were centered around a potential of approximately −1.2 V vs. Ag/AgCl consistent with the literature and expected thermodynamic potential for the Zn/ZnV redox couple.

Effect of Peak Current

The applied peak current density has been demonstrated to be a primary variable in controlling the morphology and grain size of the deposits. An increase in peak current density during the pulse plating process leads to grain refinement with grain size as small as 38 nm obtained at 1.2 A/cm². Herein it is shown that the use of applied peak current densities ranging from 32 mA/cm² to 2 A/cm² while maintaining the same pulse ON time (0.1−10 ms), OFF time (0.9−90 ms), total duration of a cycle (1 ms), duty cycle (10%), and charge density (1-50 C/cm²) is effective. In other examples the applied peak current density can be about 0.01 A/cm² to 156 A/cm² or about 0.02 A/cm² to 1.5 A/cm² and the duty cycle can be in a range of from about 0.1% to about 90% or about 2% to about 10%. FIG. 2 shows the SEM images of the deposits obtained as a function of the applied peak current density at a relatively low magnification. Optical images of the same samples are also provided as insets. All deposits were observed to be macroscopically uniform and defined precisely within the geometric area of 2.55 cm², and their thickness as measured by a stylus profiler were found to be ranging between 10 to 20 microns. Furthermore, they were found to be quite rough, with a typical Root mean square (RMS) roughness of approximately 1−3 microns for all the samples investigated in this series. While a typical stylus profilometry scan across a 500×500 μm region for the various samples revealed no major differences in roughness, the SEM images (FIG. 2) clearly show that variations in roughness do exist across regions of the sample.

As shown in FIG. 2, at low current density (0.032 A/cm²), the deposit appears uniformly covered with a granular structure. With increasing current density, the deposit becomes increasingly non-uniform, with bright secondary clusters appearing and becoming dominant at 0.5 A/cm²n. However, even higher currents of 1 and 2 A/cm² show progressively lower density of such secondary clusters. Higher magnification reveals a clear difference in micro and sub-micron structure, such that the coarse structure at low current density (0.032 A/cm²) previously identified at low magnification comprise of hexagonal-shaped plates. This morphology is consistent with prior reports from both DC and pulsed deposition methods. With increasing peak current density, a transition from these hexagon shapes to more elongated needle-like structures was clearly observed. This transition was gradual, with an initial decrease on the size of the hexagonal plates from 0.032 A/cm² to 0.5 A/cm², following which, the plate structure gave way to the elongated needle structures. Such elongated needle-shaped structures are novel, having been rarely observed in the literature only in the presence of additives such as CTAB, pyridinium, or tetrabutylammonium salts.

In addition to this underlying primary micro- and nano-structure, the samples also had a secondary deposit, which was now observable as a dendritic structure. These dendrites could explain the high RMS roughness (˜1−3 μm) observed from the 3D-scans. Furthermore, these dendrite clusters were also observed to be visibly brighter in the SEM micrographs as compared to the primary deposit, suggesting charging effects due to a more insulating nature, as compared to the material around it. Hence, energy dispersive X-Ray (EDX) analysis was performed on one of these dendritic deposits in the vicinity of the primary nanostructure. Intensity maps for Zinc and oxygen suggest that while Zinc is uniformly distributed across both the primary nanostructure and secondary dendrites, the oxygen distribution in the two have a significant difference. This, in turn, suggests partial oxidation of these dendritic structures, which is more facile than that of the primary nanostructure. This oxidation is likely a thin passivation layer, further supported by the absence of any oxide peaks in the X-ray diffraction patterns shown in FIG. 3(a).

Such observed behavior can be explained through an understanding of crystallization processes occurring during pulsed electrodeposition that has been investigated through both theoretical and experimental mechanistic investigations in the literature for zinc as well as a variety of other metals. It is hypothesized as a competition between (i) the growth of existing seeds and (ii) the generation of fresh new nuclei. This competition between crystallization modes involves three major factors, including (a) surface diffusion rates, (b) population of adsorbed atoms, and (c) applied peak current density and, consequently, the overpotential. A low surface diffusion rate combined high population of adatoms and peak current density typically favors the generation of new nuclei. However, a high surface diffusion rate combined with a low population of adatoms and peak current density favors the buildup of existing crystals. Hence, plating at low current densities (hence low overpotentials) allows for epitaxial layer by layer growth, where the migration of atoms is significant, allowing the continued growth of an existing structure. This is consistent with the large micron-sized and well-defined hexagonal plates observed at a current density of 0.032 A/cm². On the contrary, plating at high current densities (and high overpotentials) leads to increased nucleation rates and favors the formation of new grains over the growth process of existing structures. Consistent with this understanding, we also observe more dendritic structures with three-dimensional morphology as a function of increasing current density.

This series of samples were also analyzed by X-ray diffraction, and the experimental patterns are shown in FIG. 3(a) along with appropriate database standards for comparison. Primary peaks of interest marked with a blue color asterisk (*) were found at approximately 37 degrees (002 or 0002 plane), 39 degrees (100 or 1010 plane), 43.4 degrees (101 or 1011 plane), 55 degrees (102 or 102 plane), 70 degrees (103 or 1013 plane) and 71 degrees (110 or 1120 plane), identified by a match with the appropriate database standard for metallic Zinc. No oxide peaks were found as seen by comparison to the relevant database standards for zinc oxide, suggesting limited or negligible amounts of aerial oxide formation. A qualitative comparison shows a large intensity for the (002) texture of the sample at the lowest current density, which progressively becomes smaller. The (101) texture is visible in all samples but becomes increasingly dominant as compared to other textures with increasing current density. Crystallite or grain size was also determined using the Scherrer's method as previously reported in the literature. FIG. 3(b) shows the variation in crystallite size as a function of applied peak current density. Consistent with prior results, we observe a grain refinement from approximately 33 nm down to 19 nm across the range of current densities explored herein. Increased nucleation rates accompanying high peak currents have been known the lead to such a finer grain structure. The general trend of grain refinement with increasing peak current densities is consistent with prior observations, but the Scherrer's method of analysis is subject to the limitations arising from contributions of instrumental broadening, morphological effects and disorder to reflection widths. More definitive values of grain size could be determined with additional analysis through transmission electron microscopy (TEM), a subject for future studies.

Given the drastic change in the intensities of the texture from (002) to (101) as seen in FIG. 3(a), quantitative analysis was performed by calculating texture coefficients using previously reported methods. While this method is subject to notable limitations with respect to morphology and aspect ratio, it has been widely used in the literature. Since all samples investigated were thicker than 10 microns, no substantial effects of the choice of substrate are expected to hold on the resulting epitaxial growth of the deposits. Furthermore, the electrochemical parameters, such as peak current and duty cycle, are expected to dominate the growth process. FIG. 4(a) shows the variation in absolute texture coefficient (TC) as a function of varying current density. Values of TC (hk1) above unity indicates a preferred orientation of the (hk1) reflection as compared with the random distribution of grains in the reference databased standard. A substrate peak (at 43.8 degrees) marked with a red color asterisk and dotted line can also be seen in FIG. 3(a). While it is close to the (101) primary peak of interest, it is sufficiently separated to allow accurate determination of the peak intensities (not peak area) required for the determination of TC.

Consistent with qualitative trends seen in FIG. 3(a), the sample at the lowest current density comprising primarily of hexagonal plates exhibited a strong (002) texture, while all other samples demonstrated progressively increasing (101) textures. The intensity of a reflection (hk1) relative to other reflections, defined as the relative texture coefficient (RTC), was also determined and expressed as a percentage in FIG. 4(b). Again, the trend of progressively increasing (101) texture was confirmed as a function of increasing applied peak current density. Under near-equilibrium conditions (low peak currents), growth is preferred along the (002) basal plane. With increasing peak currents, the deposition process moves further and further away from equilibrium, and hence non-equilibrium structures are generated at larger angles with respect to the basal plane, as schematically shown in FIG. 5.

Furthermore, the energy of formation associated with these growth planes was identified to be (002)<(101) (110)<(100). The trend in the increasing RTC for (101) as a function of increasing peak current density is clearly demonstrated in FIG. 4(b). However, the trends for (002) and (110) are more complex involving a minimum value at ˜0.5 A/cm², which cannot be readily explained and needs further studies. The preferential growth along the high-energy planes such as (101) could also explain the needle-shaped structure observed at high current densities, viewed as an elongated form of the plate-structures shown in FIG. 5, that has grown in that preferential direction. Interestingly, the fraction of planes with the lowest (002) and highest (100) angles with respect to the basal plane (and hence the energy of formation) mirror each other as seen in FIG. 4(b), consistent with this hypothesis that the preferential growth plane varies as a function of the angle. The same trends discussed herein were found in repeated sets of samples generated under identical conditions. The dominant texture overall for samples herein were found to be either the basal (002) or the (101), depending on applied conditions. This difference in dominant texture could be attributed to the convolutive effects caused by the use of an additive (polyacrylamide or thiourea), and many other studies.

Effect of Duty Cycle

Duty cycle is defined as the ratio of time on to the total duration of the cycle, e.g., t_(ON)/(t_(ON)+t_(OFF)) expressed as a percent value. Variation in the duty cycle was achieved by holding the time on (0.1 ms), peak current density (2 A/cm²) and charge density (10 C/cm²) constant while varying the time off, resulting in values ranging from 0.1 to 50%. A lower duty cycle was achieved by increasing the off-time (t_(OFF)), which allows the diffusion layer to be replenished with ions. In comparison, a larger (%) duty cycle would imply more DC-like conditions with limited time for the replenishment of the boundary layer concentration (in this case, Zn²⁺ ions).

FIG. 6 shows the SEM images of the electrodeposited samples obtained by varying the duty cycle low (0.1%) to high (50%) values, with the range defined by the limits of the instrument used conducted at a constant current density of 2 A/cm². Different instruments can allow for different values than those listed herein This current was chosen for further investigation because of the needle-shaped morphology observed as well as the highly (101) textured surface observed in FIG. 2 (SEM) and FIG. 4 (TC analysis), respectively. Consistent with prior trends, we continued to observe a needle-shaped morphology at a high duty cycle of 50%. However, with a lower duty cycle (less than 1%), the needle shape morphology gradually gave rise to a more recognizable plate-like morphology, similar to the one observed in FIG. 2 at a much lower current density (0.032 A/cm²). An understanding of re-crystallization processes of surface diffusion and grain coalescence occurring during the off time, could explain this transition from needle-shaped back to the hexagonal plate morphology as a function of varying duty cycle. From the perspective of crystal growth processes, the same competition between (i) the growth of existing seeds and (ii) the generation of fresh new nuclei exists, as previously described. However, in this series of samples, the progressively larger off time ranging from 0.1 ms (50% duty cycle) to 99.9 ms (0.1% duty cycle) suggests the influence of re-crystallization processes wherein small thermodynamically unstable grains can coalesce to form larger ones. Furthermore, such longer off-times could also promote the migration of zinc adatoms, which can in turn promote the growth of existing crystals as opposed to the formation of fresh nucleation centers. This is further supported by an analysis of grain size as shown in FIG. 7(a), which increases with increasing off times (or lower duty cycle), consistent with the above understanding of re-crystallization processes.

FIG. 7(b) shows the variation of the RTC as a function of increasing duty cycles with all other variables held constant. At low duty cycles (<1%), the samples exhibit a highly textured (002) texture surface consistent with the idea of significant recrystallization occurring that allows the continued growth of near-equilibrium facets along the basal plane (see FIG. 5). With progressively increasing duty cycle (and correspondingly decreasing off time), the fraction of (002) texture decreases rapidly with a concurrent rise in the (101) texture.

Methods

Pulsed Electrodeposition of Zinc on Stainless steel: Substrates were obtained from McMaster Carr (0.02 inch thick). The substrates cut into approximately 1-inch×1-inch pieces using a pair of scissors. Before use, they were pre-treated by polishing with 100 and then 300 grit sandpaper, followed by ultra-sonication (Fischer brand 11205, 37 kHz) first in deionized water and then degreased in acetone. After drying the substrate, it was assembled into a custom-built parallel plate flow electrochemical cell adapted from industrial designs, previously described in the literature, serving as the working electrode (or cathode) of the cell. A freshly polished and degreased plate of pure Zinc (99.9%, STREM chemicals, 0.02-inch-thick) plate was used as the counter (anode) electrode. No reference electrode was utilized for the plating experiments, which were conducted galvanostatically in a two-electrode configuration. The electrolyte used for plating was an aqueous solution (Type-I deionized water with resistivity>18 MΩ cm) containing 0.1 M Zinc Sulphate Pentahydrate (M. W. 251.5) and 0.9 M Sodium Sulphate (M.W. 142.04, Sigma Aldrich) with a pH of 5.34 (Acumet AE105, Fischer scientific). This liquid electrolyte was circulated in a closed loop between the flow cell and the 25-mL reservoir at a constant volumetric flow rate of 1 mL/min using a peristaltic pump (Masterflex L/S Series). All tubing connections were coupled together with PFA or stainless-steel compression fittings (Swagelok). The electrolyte was saturated and continuously purged with Nitrogen gas (Mississippi Welders, 99.99% UHP) for 15 minutes prior to electroplating and maintained throughout the duration of the experiment with a fresh batch of electrolyte used for each experiment. The active area for deposition was confined to 1 5×1.7 cm defined precisely using rubber gaskets providing an active geometric deposition area of 2.55 cm². Electroplating was conducted by application of pulsed waveforms from a pulse rectifier (Dynatronix Microstar Series—XR). A Tektronix TDS-1002 Oscilloscope was used to monitor the fidelity of the waveform generated by the pulse rectifier and ensure that it remained in a rectangular shape with no observable damping. After plating, the coated substrates were removed from the cell, gently rinsed with deionized water, and left to dry overnight inside a dry box continuously purged with dry Nitrogen. All experiments were conducted under ambient conditions at approximately 20±1° C. and performed at least twice to ensure reproducibility.

Scanning electron microscope (SEM) images were acquired using a Zeiss EVO HD with a Bruker Energy-dispersive spectrometer at 20 kV acceleration voltage. Thickness was measured using Tencorp P-7 Stylus profiler. Optical images were taken using an AF202 digital inspection camera (Amscope). X-ray diffraction was conducted using a Siemens D500 diffractometer fitted with a Cu K-alpha source (A=0.15406 nm). Texture coefficients were calculated as per literature, using the formula:

$C_{hkl} = \frac{I_{{({hkl})}i}/I_{0{({hkl})}i}}{\frac{1}{n}*{\sum_{n}{I_{{({hkl})}n}/I_{0{({hkl})}n}}}}$

Where C_(hk1) is the texture coefficient of the facet (hk1), I_((hk1)) is the intensity of the (hk1) reflection of the sample under analysis, I_(0(hk1)) is the standard intensity of the (hk1) reflection of a sample taken from a database standard (JCPDS 04-0831), and n is the number of reflections taken into account. If the value of the texture coefficient is higher than unity, it means the particular facet is preferentially oriented, Relative texture coefficients (RTC) were calculated using the formula:

${{RTC}_{hkl}(\%)} = {\left( \frac{C_{hkl}}{\sum_{n}C_{hkl}} \right)*100}$

The average grain or crystallite size was calculated as per Scherrer's method using the formula:

$D = \frac{K\Lambda}{\beta\cos\theta}$

Where, D is the crystallite size in nm, K (0.9) is the Scherrer constant, A is the wavelength of Cu

K-Alpha X-rays, β is the Full width half maximum (radians) or FWHM, and Θ is the peak position (radians).

Example 2: Deposition of Zinc on Carbon Steel Substrate Materials and Experimental Procedures

Carbon steel substrates were obtained from McMaster Carr and cut into approximately 1 inch×1 inch square pieces. Samples were polished mechanically using standard metallographic techniques using a Nano1000S grinder/polisher (PACE technologies). Successive polishing was done using sanding paper of 240, 360, 600, 800 and 1200 grit SiC paper rotating at 200 rpm and continuous flowing water as per manufacturer recommendations. Final polishing was done using a 1200 grit SiC paper rotating at 200 rpm with 0.05 um Alumina slurry as an abrasive for 1 minute to obtain a mirror finish. After polishing, the substrate was placed in an ultrasonic bath with a 50-50 acetone-deionized (DI) water mixture for 15 minutes at 37 kHz, 298K. Finally, the sample was rinsed with DI water and dried under a stream of compressed air.

Electrodeposition

Pre-treated substrates were assembled into a custom-built parallel plate flow electrochemical cell adapted from industrial designs serving as the working electrode (or cathode) of the cell. A freshly polished and degreased plate of pure Zinc (99.9%, STREM chemicals, 0.02-inch-thick) plate was used as the counter (anode) electrode. No reference electrode was utilized for the plating experiments, which were conducted galvanostatically in a two-electrode configuration. The electrolyte used for plating was an aqueous solution (Type-I deionized water with resistivity>18 MΩ.cm) containing 0.1 M Zinc Sulphate Pentahydrate (M.W. 251.5) and 0.9 M Sodium Sulphate (M.W. 142.04, Sigma Aldrich) with a pH of 5.34 (Acumet AE105, Fischer scientific). This liquid electrolyte was circulated in a closed loop between the flow cell and the 25-mL reservoir at a constant volumetric flow rate of 1 mL/min using a peristaltic pump (Masterflex L/S Series). All tubing connections were coupled together with PFA or stainless-steel compression fittings (Swagelok). The electrolyte was saturated and continuously purged with Nitrogen gas (Mississippi Welders, 99.99% UHP) for 15 minutes prior to electroplating and maintained throughout the duration of the experiment with a fresh batch of electrolyte used for each experiment. The active area for deposition was confined to 1.5×1.7 cm defined precisely using rubber gaskets providing an active geometric deposition area of 2.55 cm². Electroplating was conducted by application of pulsed waveforms from a pulse rectifier (Dynatronix Microstar Series—R). A Tektronix TDS-1002 Oscilloscope was used to monitor the fidelity of the waveform generated by the pulse rectifier and ensure that it remained in a rectangular shape with no observable damping. After plating, the coated substrates were removed from the cell, gently rinsed with deionized water, and left to dry overnight inside a dry box continuously purged with dry Nitrogen. All experiments were conducted under ambient conditions at approximately 20 ±1° C. and performed at least twice to ensure reproducibility. This example explores the use of applied peak current densities ranging from 32 mA/cm² to 2 A/cm² while maintaining the same pulse ON time (0.1−10 ms), OFF time (0.9-90 ms), total duration of a cycle (1 ms), duty cycle (10%), and charge density (1-50 C/cm²). Furthermore, duty cycle (0.1 to 50%) and charge density (10-30 C/cm²) were also varied to investigate their role on the deposition process, while keeping all other parameters constant. In other examples the peak current density can be about 0.01 A/cm² to 156 A/cm² or about 0.02 A/cm² to 1.5 A/cm² and the duty cycle can be in a range of from about 0.1% to about 90% or about 2% to about 10%. This report focuses on the role of duty cycle on texture and consequently corrosion resistance.

Corrosion Testing and Analysis

A three-electrode single-compartment (undivided) cell was used for all corrosion tests. It included a working electrode, an Ag/AgCl reference electrode, and an iridium oxide counter electrode; the working area was defined as 1.15 cm×1.15 cm. Corrosion rates were calculated through linear Polarization Resistance (LPR) tests using 0.1 M aqueous NaCl purged with inert gas (Ar) at 10 cm, connected through the bottom inlets. For each electrochemical test, ASTM certified procedure as described in G59-97 (“Standard Test Method For Conducting Potentiodynamic Polarization Resistance Measurements”, 2020) were adopted briefly summarized as followed: the open circuit voltage (OCV) was monitored for 55 minutes, during which minimal fluctuations (<10 mV) were observed.

A stepped potential −30 mV (−0.030 V) w.r.t to the OCV was maintained for 1 min, after which the potential was swept positive at a rate of 10 mV/min (0.6 V/hour) until+30 mV (+0.030 V) w.r.t the original OCV, followed by termination of the experiment. Corrosion currents (I_(corr), reported as μA/cm²) were determined using the formula (1) where β_(a) and β_(c) are the Tafel constants and R_(p) is the resistance to polarization. Tafel constants and R_(p)(Ohms) was calculated using the EC-Lab software (Biologic LLC) from polarization curves, as per the Stern-Geary equation (I)

$\begin{matrix} {I_{corr} = \frac{\beta_{a}*\beta_{c}}{2.303*R_{p}*\left( {\beta_{a} + \beta_{c}} \right)}} & (1) \end{matrix}$

The corrosion current was further converted into a corrosion rate using equation (2), where EW is the equivalent weight of the metal undergoing corrosion, and I_(corr) expressed in μA/cm².

$\begin{matrix} {{{Corrosion}{Rate}\left( {{mm}/{py}} \right)} = \frac{I_{corr}*{EW}*3.27 \times 10^{- 3}}{Density}} & (2) \end{matrix}$

Results

Initial electroplating experiments were conducted with variation of either (i) peak current density or (ii) duty cycle and XRD was used to assess the changes in texture. The latter (duty cycle) while maintaining high peak current density of 2 A/cm² was found to generate the largest contrast in texture across the range of values explored, and hence was chosen for further testing. We observed a transition of morphology from commonly observed hexagonal plates (at low duty cycle) of Zinc to the rarer needle-shaped deposits (at high duty cycle). Such a change in morphology is accompanied by a change in preferred crystalline orientation observed from XRD analysis, and is consistent with prior results demonstrated in stainless steel substrates.

FIG. 8 shows the SEM images of the deposits obtained on carbon steel as a function of the duty cycle at various magnifications. Optical images of the same samples are also provided as insets. All deposits were observed to be macroscopically uniform and defined precisely within the geometric area of 2.55 cm². Duty cycle is defined as the ratio of time on to the total duration of the cycle, e.g., t_(ON)/(t_(ON)+t_(OFF)) expressed as a percent value. Variation in the duty cycle was achieved by holding the time on (0.1 ms), peak current density (2 A/cm²), constant while varying the time off, resulting in values ranging from 0.1 to 50%. A lower duty cycle was achieved by increasing the off-time (t_(OFF)), which allows the diffusion layer to be replenished with ions. In comparison, a larger (%) duty cycle would imply more DC-like conditions with limited time for the replenishment of the boundary layer concentration (in this case, Zn²⁺ ions).

FIG. 9 shows the variation of the relative texture coefficient as a function of increasing duty cycles with all other variables held constant. At low duty cycles (0.1%), the samples exhibit a highly textured (002) texture surface consistent with the idea of significant recrystallization occurring that allows the continued growth of near-equilibrium facets along the basal plane—this has been previously seen with stainless steel substrates as well. With progressively increasing duty cycle (and correspondingly decreasing off time), the fraction of (002) texture decreases rapidly with a concurrent rise in the (101) texture. Grain sizes of these samples were also calculated as per the Scherrer equation to be 34 nm (0.1% duty cycle) and 28 nm (50% duty cycle) respectively.

FIG. 10(a) shows the polarization curves for the two zinc coated carbon steel substrates generated at low (0.1%) and high (50%) duty cycle in a de-aerated 0 M aqueous NaCl electrolyte, chosen as a typical corrosion media found in many applications. Note the lower current range observed for the former sample (0.1% duty cycle) as compared to the latter (50% duty cycle) indicating a difference in corrosion currents. These polarization cures were then used to obtain corrosion rates described in mm/yr as per standard ASTM certified methods. FIG. 10(b) shows the corrosion rates for the two zinc coated carbon steel substrates generated at low (0.1%) and high (50%) duty cycle. Note the break in the y-axis required to accommodate all the values therein. The uncoated stainless steel (literature standard) has by far the lowest corrosion rate (6.7×10⁻⁴ mm/yr) as expected, given the addition of chromium to the steel composition. However, this addition makes it less cost effective and thus not suitable for many commercial applications. This necessitates the use of carbon steel, which has a much higher corrosion rate of approximately 0.2 mm/yr. Finally, the zinc coated carbon steel samples generated herein show a corrosion rate of 5.7×10⁻³ mm/yr (0.1% duty cycle) and 1.9×10⁻² mm/yr (50% duty cycle) respectively, which are comparable to those shown by stainless steel. Further comparing these values shows that the low duty cycle (0.1%) which exhibited a dominant (002) texture (see FIG. 9) indeed exhibits a lower corrosion rate (approx. 3-fold difference) as compared to the sample generated at high duty cycle (50%), which is consistent with literature precedent. However, given the simplicity of the plating bath used herein to deposit zinc implies that an effective corrosion resistance can be achieved without the use of any additive-based formulations.

The terms and expressions that have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the embodiments of the present invention. Thus, it should be understood that although the present invention has been specifically disclosed by specific embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those of ordinary skill in the art, and that such modifications and variations are considered to be within the scope of embodiments of the present invention.

Exemplary Aspects

The following exemplary aspects are provided, the numbering of which is not to be construed as designating levels of importance:

Aspect 1 provides a method of electrodeposition of zinc, the method comprising:

independently controlling at least one of an electrical peak current and a duty cycle; and

depositing the zinc on a substrate.

Aspect 2 provides the method of Aspect 1, wherein the electrical peak current has a density is in a range of from about 32 mA/cm² to 156 A/cm².

Aspect 3 provides the method of Aspect 2, wherein the electrical peak current has a density is in a range of from about 0.02 A/cm² to 1.5 A/cm².

Aspect 4 provides the method of Aspect 1, wherein the duty cycle is in a range of from about 0.1% to about 90%.

Aspect 5 provides the method of Aspect 4, wherein the duty cycle is in a range of from about 2% to about 10%.

Aspect 6 provides the method of Aspect 1, wherein the electrical peak current has a density in a range of from about 0.02 A/cm² to 1.5 A/cm² and the duty cycle is in a range of from about 2% to about 10%.

Aspect 7 provides the method of Aspect 1, wherein the electrical peak current has a density in a range of from about 0.02 A/cm² to 0.5 A/cm² and the duty cycle is in a range of from about 5% to about 10%.

Aspect 8 provides the method of Aspect 1, wherein the substrate comprises steel.

Aspect 9 provides the method of Aspect 8, wherein the steel comprises stainless steel, carbon steel, or a combination thereof.

Aspect 10 provides the method of Aspect 1, wherein at least one of the electrical peak current and the duty cycle are independently controlled such that the zinc layer comprises a plurality of needle-shaped structures, hexagonal-plate structures, or a mixture thereof.

Aspect 11 provides a method of electrodeposition of zinc, the method comprising:

independently controlling at least one of an electrical peak current and a duty cycle wherein the electrical peak current has a density in a range of from about 0.02 A/cm² to 1.5 A/cm² and the duty cycle is in a range of from about 2% to about 10%; and

depositing the zinc on a substrate, wherein the zinc layer comprises a plurality of needle-shaped structures, hexagonal-plate structures, or a mixture thereof.

Aspect 12 provides an assembly comprising:

a steel substrate; and

a zinc layer deposited about the steel substrate, wherein the zinc layer comprises a plurality of needle-shaped structures, hexagonal-plate structures, or a mixture thereof.

Aspect 13 provides the assembly of Aspect 12, wherein the steel substrate comprises stainless steel, carbon steel, or a combination thereof.

Aspect 14 provides the assembly of Aspect 12, wherein the steel substrate is substantially planar.

Aspect 15 provides the assembly of Aspect 12, wherein the zinc layer comprises a plurality of needle-shaped structures.

Aspect 16 provides the assembly of Aspect 12, wherein the zinc layer comprises a plurality of hexagonal-plate structures.

Aspect 17 provides the assembly of Aspect 12, wherein the plurality of needle-shaped structures, hexagonal-plate structures, or a mixture thereof are oriented.

Aspect 18 provides the assembly of Aspect 12, wherein the plurality of needle-shaped structures, hexagonal-plate structures, or a mixture thereof comprise an average crystallite size in a range of from about 10 nm to about 50 nm.

Aspect 19 provides the assembly of Aspect 18, wherein the plurality of needle-shaped structures, hexagonal-plate structures, or a mixture thereof comprise an average crystallite size in a range of from about 19 nm to about 33 nm.

Aspect 20 provides the assembly of Aspect 12, formed by the method of Aspect 1. 

1. A method of electrodeposition of zinc, the method comprising: independently controlling at least one of an electrical peak current and a duty cycle; and depositing the zinc on a substrate to form a zinc layer.
 2. The method of claim 1, wherein the electrical peak current has a density is in a range of from about 32 mA/cm² to 156 A/cm².
 3. The method of claim 2, wherein the electrical peak current has a density is in a range of from about 0.02 A/cm² to 1.5 A/cm².
 4. The method of claim 1, wherein the duty cycle is in a range of from about 0.1% to about 50%.
 5. The method of claim 4, wherein the duty cycle is in a range of from about 2% to about 90%.
 6. The method of claim 1, wherein the electrical peak current has a density in a range of from about 0.02 A/cm² to 1.5 A/cm² and the duty cycle is in a range of from about 2% to about 10%.
 7. The method of claim 1, wherein the electrical peak current has a density in a range of from about 0.02 A/cm² to 0.5 A/cm² and the duty cycle is in a range of from about 5% to about 10%.
 8. The method of claim 1, wherein the zinc is present in a plating bath solution that is free of any additives.
 9. The method of claim 1, wherein the steel comprises stainless steel, carbon steel, or a combination thereof.
 10. The method of claim 1, wherein at least one of the electrical peak current and the duty cycle are independently controlled such that the zinc layer comprises a plurality of needle-shaped structures, hexagonal-plate structures, or a mixture thereof.
 11. A method of electrodeposition of zinc, the method comprising: independently controlling at least one of an electrical peak current and a duty cycle wherein the electrical peak current has a density in a range of from about 0.02 A/cm² to 1.5 A/cm² and the duty cycle is in a range of from about 2% to about 10%; and depositing the zinc on a substrate, wherein the zinc layer comprises a plurality of needle-shaped structures, hexagonal-plate structures, or a mixture thereof.
 12. An assembly comprising: a steel substrate; and a zinc layer deposited about the steel substrate, wherein the zinc layer comprises a plurality of needle-shaped structures, hexagonal-plate structures, or a mixture thereof.
 13. The assembly of claim 12, wherein the steel substrate comprises stainless steel, carbon steel, or a combination thereof.
 14. The assembly of claim 12, wherein the steel substrate is substantially planar.
 15. The assembly of claim 12, wherein the zinc layer comprises a plurality of needle-shaped structures.
 16. The assembly of claim 12, wherein the zinc layer comprises a plurality of hexagonal-plate structures.
 17. The assembly of claim 12, wherein the plurality of needle-shaped structures, hexagonal-plate structures, or a mixture thereof are oriented.
 18. The assembly of claim 12, wherein the plurality of needle-shaped structures, hexagonal-plate structures, or a mixture thereof comprise an average crystallite size in a range of from about 10 nm to about 50 nm.
 19. The assembly of claim 18, wherein the plurality of needle-shaped structures, hexagonal-plate structures, or a mixture thereof comprise an average crystallite size in a range of from about 19 nm to about 33 nm.
 20. The assembly of claim 12, formed by the method of claim
 1. 